Biological Control of HWA with Sasajiscymnus tsugae
Sasi adult feeding on Adelgid ovisac |
How does biological control work?
Biological control of HWA requires introducing predator organisms that can reduce HWA populations to a level that does not harm our native hemlock species. The Sasi predator beetle is the most effective biocontrol option for private property owners (see The Case for Sasajiscymnus tsugae). Over 3 million Sasi beetles produced by taxpayer-supported USDA labs have been released in national and state parks and forests. But all other hemlocks are dependent on the efforts of private individuals and groups (and purchases from private labs) to implement the biological control strategies needed to protect our hemlock ecosystems.
HWA kills by infesting and defoliating our native hemlocks. Adelgid feeding dessicates the individual needles and heavy HWA infestation suppresses the production of new hemlock foliage ( Introduction to Biological Control of Hemlock Woolly Adelgid). So the trees die from the loss of foliage needed for water take-up and food production. Sasi interrupts this cycle by dramatically reducing adelgid populations, allowing hemlocks to produce new foliage and gradually recover. Both adult Sasi beetles and larvae are voracious HWA predators. And because these specialized beetles cannot reproduce without an adelgid food source, they have been judged (by USDA) to not be a threat to other native US insect populations. (See photo of Sasi beetle feeding on Adelgid ovisac above.)
Where did HWA come from?
The HWA that is devastating our eastern hemlocks was introduced from Japan to Richmond, VA on private nursery stock. It was identified by USDA in 1951, then allowed to spread unchecked across the eastern US for 40 years (see historical map of infestation). Biological control efforts for this HWA were initiated by USDA in 1991, seeking to locate and test (under quarantine) predator organisms that could control HWA population growth. (Note that the HWA found in the Pacific Northwest is a different adelgid, not a recent introduction – and both western and eastern hemlock species show considerable resistance to this HWA.)
In its Japanese homeland area, our HWA "import" causes no damage to the native hemlocks (Tsuga sieboldii). Those Japanese hemlocks are very similar to our eastern hemlocks in having low levels of genetic resistance to HWA (see technical paper Resistance of Hemlock Species to HWA). And the native predator beetle that controls this HWA in Japan is a small, black ladybird beetle named Sasajiscymnus tsugae (Sasi or St for short). (Until 2004 it was called Pseudoscymnus tsugae or Pt - same beetle, different name.)
How effective is Sasi?
In 1995 USDA approved Sasi beetles for field release, and careful field research efforts at research facilities in CT have established the effectiveness of Sasi as an HWA predator (see Biological Control Agents for Hemlock Woolly Adelgid). These research results include up to 80% adelgid reductions in a single season, a 2 generation-per-year (bivoltine) reproductive cycle that allows Sasi to match the exponential growth capability of HWA, an ability to match HWA in over-wintering in cold winter climates, and an ability to move (and expand coverage) considerable distances over the course of a single season. And a research assessment, comparing treatment and control sites 4 years after Sasi releases, found that hemlock foliage densities were significantly better where beetles were released. (see Assessments of Biological Control of HWA with Sasajiscymnus tsugae in Connecticut and New Jersey)
My own field-testing of Sasi predator beetle releases in western NC began on my own property in 2006 and has continued on a variety of public and private properties in 2007 - 2009 (about 100,000 privately released beetles through 2009). In the process I have developed and tested low-density release procedures that provide 10 times the area coverage of the USDA release protocols (or 10% of the cost for a small area). This makes biological control a cost-effective strategy for private property owners. These private biocontrol efforts have also initiated a growing body of observations and knowledge about beetle behavior and assessment methods (see Predator Beetles at Work - Evidence based Assessment of Private Sasjiscymnus tsugae Release Sites in Wester North Carolina).
A grass-roots community effort in the Brevard NC area has involved city and county governments, conservation groups, summer camps, neighborhood associations and individual landowners in an HWA biological control effort that should safeguard the future health of hemlocks in the "greater Brevard area". After 3 years of Sasi releases in Brevard, hemlocks in most parts of the city have “turned the corner” and are recovering - with increasing new foliage production and reduced adelgid populations. So Brevard will be the first US community to experience “beetle-assisted recovery” of its resident hemlock population.